early 15c., "a restoring to a former state; a subjugation" (of a people, etc.), from Middle French reducion (13c., Modern French réduction) and directly from Latin reductionem (nominative reductio) "a leading back, restoration," noun of action from past participle stem of reducere (see reduce). Meaning "diminution, a lessening" is from 1670s; chemical sense of "reversion to a simpler form" is from 1660s.

The changing of a fraction into a simpler form, especially by dividing the numerator and denominator by a common factor. For example, the fraction 8/12 can be reduced to 4/6 , which can be further reduced to 2/3 , in each case by dividing both the numerator and denominator by 2.

A chemical reaction in which an atom or ion gains electrons, thus undergoing a decrease in valence. If an iron atom having a valence of +3 gains an electron, the valence decreases to +2. Compare oxidation.

Our Living Language: Beginning students of chemistry are understandably puzzled by the term reduction: shouldn't a reduced atom or ion be one that loses electrons rather than gains them? The reason for the apparent contradiction comes from the early days of chemistry, where reduction and its counterpart, oxidation, were terms invented to describe reactions in which one substance lost an oxygen atom and the other substance gained it. In a reaction such as that between two molecules of hydrogen (2H2) and one of oxygen (O2) combining to produce two molecules of water (2H2O), the hydrogen atoms have gained oxygen atoms and were said to have become "oxidized," while the oxygen atoms have (as it were) lost them by attaching themselves to the hydrogens, and were said to have become "reduced." Importantly, though, in the process of gaining an oxygen atom, the hydrogen atoms have had to give up their electrons and share them with the oxygen atoms, while the oxygen atoms have gained electrons. Thus comes the apparent paradox that the "reduced" oxygen has in fact gained something, namely electrons. Today the terms oxidation and reduction are used of any reaction, not just one involving oxygen, where electrons are (respectively) lost or gained.